With Bill Clinton being named in an explosive new cheating scandal, The National ENQUIRER can exclusively reveal the identity of his “on-off mistress” had been wealthy blonde divorcée Julie Tauber McMahon!

The ex-president first pursued Julie after his wife, Hillary, was elected U.S. senator from New York in 2000 and moved to Washington, D.C., part-time, as we’ve reported in the past.

Insiders say the illicit relationship cooled for a time, but heated up again about three years ago.

“Hillary heard Bill put Julie on hold while she was campaigning for president in 2008, but he went back to her in the months after their daughter, Chelsea, got married in July 2010,” a close source told The ENQUIRER.

Author Ronald Kessler claims in a new book that Bill – who turns 68 on August 19 – is currently two-timing Hillary with “a buxom blonde mistress” known to his Secret Service detail as the “Energizer.”

According to his book, “The First Family Detail: Secret Service Agents Reveal the Hidden Lives of the Presidents,” Bill’s minders hid the Energizer from Hillary, and the Energizer occasionally rewarded them with cookies.

The ENQUIRER contacted Ronald to ask if Julie, a 54-year-old mother of three, is the Energizer, but he refused to comment.

An insider said: “There’s questions now whether the ‘Energizer’ Kessler is referring to is, in fact, Julie Tauber McMahon.”

Meanwhile, insiders have told The ENQUIRER that Bill first met Julie in 1998 through her father, Michigan millionaire Joel Tauber, then a top Democratic Party contributor and Clinton family friend.

Although Julie has denied a romantic relationship with the former Horndog-in-Chief, one of her relatives, who passed a polygraph test, confirmed it in 2011.

“It became a running joke in the family,” the source told The ENQUIRER. “If Clinton traveled to London, we’d learn Julie was also visiting London. When Clinton went to France, we’d discoverJulie was also in France.”

The couple regularly sneaked into each other’s homes – they live just five miles apart in Chappaqua, N.Y. – for trysts. The source said: “They even slept in Hillary’s bed.”

Julie’s father disapproved of the affair and she eventually dumped Bill, added the source.

“But they couldn’t stay away from each other,” said the source. “Around the time Chelsea got married, he got back together with Julie.”

In a written statement to The ENQUIRER on March 3, 2011, Julie wrote: “My reply remains the same, we are friends and the allegations are false.”

It’s unclear what the status of Bill’s relationship with Julie has been from 2011 until now.

The ENQUIRER visited Chappaqua in late July and discovered Bill took “his girlfriend” to a local jewelry store, Desires By Mikolay, and bought a bracelet for her.

Around the same time, another eyewitness spotted Bill dining with a “honey blonde” at local French bistro Le Jardin du Roi.